Transcobalamin II (TC II) is essential, presumably through its role in vitamin B-12 (B-12) transport, in the humoral antibody response in man. In the congenital absence of TC II resistence to infection is much impaired. It is proposed to study first the need and consequence of TC II-B12 entry into stimulated human lymphocytes. Next to be studied are the roles of B-12 and TC II entry on the capacity of the lymphocytes to produce immunoglobulins. Should a defect be observed in the absence of TC II-B12, the mechanism will be explored.